Several types of catheters are made of a material which is radiopaque, so that the catheter is visible under fluoroscopy or other form of x-ray diagnosis. Typically, catheters for the arteriovenus system are made radiopaque generally by compounding into the plastic material of the catheter a radiopaque filler such as bismuth subcarbonate, bismuth trioxide, or barium sulfate.
Difficulties arise with thin-walled catheters, for example the well-known P.T.C.A. guiding catheters. These catheters are generally thin-walled since they are for the purpose of guiding another device thru its lumen. The lumen must be sufficiently large to allow the injection of contrast media around this second device (typically a PTCA dilatation catheter). As such, these thin walled catheters do not show up well on the fluoroscope or other forms of x-ray, even when they are loaded as much as possible with a radiopaque agent. The loading of radiopaque agent has an upper limit which is governed in part by the desired physical characteristics of the catheter. Plastic materials with heavy loadings of radiopaque agent decrease in flexibility, which may limit their use at higher concentrations in many types of catheters.
It is particularly desirable for the distal tip of the catheter to be visible in a fluoroscope or other form of x-ray, so that the positioning of the catheter at its distal end can be precisely determined. In the prior art, this has been accomplished by providing a metal ring to the catheter adjacent the distal end. It is generally undesirable to place the metal ring exactly on the distal tip of the catheter, since the distal tip needs to be very soft and pliable. However, when the metal ring is spaced from the distal tip, as is conventional, it still provides a rigid section of the catheter which can be undesirable. Also, since the metal ring is spaced from the distal tip, the use of such a metal ring does not completely resolve the problem of precisely locating the distal tip of the catheter within the body by means of a fluoroscope during a medical procedure, since the metal ring is and must be spaced from the distal tip.
In accordance with this invention, a catheter is provided which is of the appropriate stiffness throughout, and preferably free from any metal radiomarker member such as a metal ring, yet which carries a distal tip which is considerably softer and more radiopaque than the rest of the catheter. Thus, the catheter may be as stiff as desired, but the highly radiopaque tip of the catheter can be soft to avoid vessel trauma, while providing reliable locatability for the distal end by fluoroscope or the like. At the same time the desired degree of flexibility is provided along every portion of the catheter, including the distal tip and the catheter segment where in the prior art a metal ring is normally carried to achieve locatability.